Fix unable to parse error(As reported by SJoseph)

Ignore EOFError raised by stderrout.read_nonblock, since with
sufficiently big stdout, small stderrout and small buffer_size, err
will reach EOF before the stdout is completely read. This would
cause the graders output to be truncated.
Remove unnecessary comments.
Merge branch 'unable_to_parse_fix' into per_part_due_date_and_grace_period